Research On Sorption And Mechanism Of Typical Organic Pollutants And Heavy Metal Ions By Microplastics

The pollution of microplastics in the environment has received extensive attention in recent years.Considering their high hydrophobicity and large specific surface area,microplastics can strongly absorb pollutants in the environment,which can affect the migration,transformation and biological effectiveness of pollutants,thereby bringing risks to the ecological environment.In this paper,microplastics including chlorinated polyethylene(CPE),polyvinyl chloride(PVC),high-density polyethylene(HPE)and low-density polyethylene(LPE₁,LPE₂)were selected as sorbents.The sorption behavior of typical organic pollutants including 1,2,3,4-tetrachlorobenzene(1,2,3,4-Te CB),phenanthrene(PHEN),naphthalene(NAPH),2,4-dinitrotoluene(2,4-DNT),2,4-dichlorophenol(2,4-DCP),tricyclazole(TRI)and heavy metal ions(Pb²⁺,Cu²⁺,Cd²⁺,Cr₂O₇^2-)on microplastics and the influencing factors were investigated.The mechanisms of sorption of different pollutants to microplastics were explored.The influence and interaction of dissolved organic matter(DOM)on the sorption of typical pollutants by microplastics was studied.This study aims to provide basic data for revealing the influence of microplastics on the migration and transformation process of pollutants in the aquatic environment.The main findings are as follows:(1)The characterization results of microplastics showed that the molecular weights of five microplastics ranged from 2.84×10³ to 2.87×10⁵.The crystallinities of LPE₁,LPE₂ and HPE were 38.1,45.8 and 77.9%,respectively.CPE was a non-crystalline substance,and PVC was a low-crystalline substance.The order of the specific surface area of different microplastics was PVC(8.9 m²/g)>LPE₂(5.6 m²/g)>HPE(3.1 m²/g)>CPE(2.3 m²/g)>LPE₁(1.3 m²/g).Due to the existence of Cl-,the surface of CPE and PVC was more negatively charged.The surface of HPE was oxidized.(2)The sorption isotherms of six organic pollutants on microplastics can be fitted with the Freundlich model.The sorption strength followed the order of 1,2,3,4-Te CB>PHEN>NAPH>2,4-DCP>2,4-DNT>TRI.The K_D(microplastics-water partition coefficient)was positively correlated with the K_OW(octanol-water partition coefficient)of organic pollutants.The sorption capacity of microplastics for organic pollutants followed the sequence of CPE>PVC>LPE>HPE.The crystallinity of microplastics was the main factor affecting the sorption of organic pollutants.The K_D value of polyethylene with different crystallinity to sorb organic pollutants was inversely proportional to the crystallinity.The specific surface area and particle size of microplastics had little effect on the sorption of organic pollutants.The mechanism analysis showed that hydrophobic partitioning was the dominant mechanism in the sorption of organic pollutants by microplastics.(3)The sorption of heavy metal ions on the four microplastics can be fitted by Freundlich and Langmuir models.The sorption strength followed the sequence of Pb²⁺>Cu²⁺>Cd²⁺>Cr₂O₇^2-.The sorption capacity of microplastics for heavy metal ions followed the order of CPE>PVC>HPE>LPE.It was speculated that the Cl-of CPE and PVC was beneficial to the sorption of heavy metal ions.The sorption of Pb²⁺,Cu²⁺and Cd²⁺by microplastics strengthened with increased p H(3.0?6.5).When the concentration of Na⁺was in the range of 0.01?1.0 mol/L,ionic strength had a weak inhibitory effect on the sorption of Pb²⁺on microplastics,while it had a significant inhibitory effect on the sorption of Cu²⁺and Cd²⁺on PVC and LPE,and the sorption of Cd²⁺on CPE.Ultraviolet aging can oxidize CPE,enhance the negative charge of the surface and the complexation with heavy metal ions,thereby increasing the sorption capacity.The sorption capacity of binary mixtures of heavy metal ions(Cu-Cd,Cu-Pb,Cd-Pb)on CPE was lower than that of individual metal ions.The coexisting ions had an inhibitory effect,among which the mutual inhibition between Cu²⁺and Cd²⁺,Cu²⁺and Pb²⁺was obvious,while the mutual inhibition between Pb²⁺and Cd² was weak.The mechanism analysis showed that the sorption of Pb²⁺on microplastics was mainly affected by electrostatic interaction,while for Cu²⁺and Cd²⁺,electrostatic interaction,together with surface complexation,determined the sorption process.(4)The study of the influence of DOM on the sorption of heavy metal ions on microplastics showed that the DOM₁ produced by water extraction was mainly fulvic acid,which contained a large number of carboxyl,hydroxyl and other oxygen-containing functional groups.The main components of DOM₂ and DOM₃ prepared by alkaline extraction silica gel chromatography were humic acids,whose structure was mainly fatty chain.Through the analysis of spectral characteristics,it was proved that the hydrophobic interaction between DOM and microplastics was dominant,and there was electrostatic interaction or complexation between DOM and heavy metal ions.By simulating the sorption of DOM(tannic acid and gallic acid)on microplastics,it was further proved that the interaction mechanism of DOM and CPE included hydrophobic interaction and electrostatic interaction.The interaction mechanism of DOM,PVC and LPE was mainly hydrophobic interaction.DOM₁ inhibited the sorption of Pb²⁺,Cu²⁺and Cd²⁺on CPE,and the inhibitory effect increased with the increase of their concentration(2.0?200.0mg/L).DOM₁ promoted the sorption of three metal ions on PVC,HPE and LPE at low concentration(2.0 mg/L),and inhibited the sorption at high concentrations(20.0?200.0 mg/L).Additionally,at low concentration(2.0 mg/L),DOM₂,DOM₃,tannic acid and gallic acid inhibited the sorption of Pb²⁺,Cu²⁺and Cd²⁺on CPE and promoted the sorption on PVC and LPE.Mechanism analysis showed that DOM inhibited the sorption of heavy metal ions on microplastics by reducing the surface electronegativity of the microplastics(the CPE surface had a strong electronegativity,so the inhibitory effect was particularly obvious).DOM promoted the sorption of heavy metal ions on the microplastics by increasing the complexing sites on the surface of the microplastics or forming a ternary complex of microplastics-DOM-metal ions.(5)Research on the influence of DOM on the sorption of phenanthrene(PHEN)on CPE showed that under the presence of DOM₁,DOM₂ and DOM₃,the K_D values of PHEN on CPE were 4995,16641 and 20704 L/kg,respectively,which were lower than that of PHEN on CPE(32385 L/kg),indicating that the DOM can inhibit the sorption of PHEN on CPE,and DOM₁had the strongest inhibitory effect.According to the theoretical calculation of site energy distribution,DOM shifted the energy distribution range of PHEN sorbed on CPE to the left,and reduced the sorption site and the sorption amount.The mechanism analysis showed that DOM can inhibit the sorption of PHEN by competing with PHEN for the sorption sites on the microplastics,and by increasing the water solubility of PHEN.